scrapy.core.downloader.handlers.http11.ScrapyAgent and scrapy.core.downloader.handlers.http11.TunnelingAgent take some args (contextFactory in both and crawler in ScrapyAgent) that are currently declared optional but in fact need to be passed as the code assumes they aren't None. So ideally they should be declared as required (but that requires reordering args and/or making all args of those constructors keyword-only so there is no clear and simple fix).